1. Field of the Invention
Example embodiments of the present patent application relate to a cleaning unit, a process cartridge incorporating the cleaning unit, and an electrophotographic image forming apparatus incorporating the cleaning unit, and more particularly, to a cleaning unit used to clean an image carrier having an electrostatic latent image formed thereon by electrophotographic, electrostatic recording, or electrostatic printing methods and the like employed in a process cartridge and an electrophotographic image forming apparatus such as a copier, printer, facsimile machine, etc.
2. Discussion of the Related Art
Electrophotography is an image forming method for forming electrostatic latent images by using an image carrier, a charging unit, an exposure unit, a developing unit, a transfer unit, and a cleaning unit. Firstly, the charging unit uniformly charges the surface of the image carrier to a positive or negative polarity and the exposure unit exposes the surface of the image carrier according to image data. The developing unit then develops the electrostatic latent image formed on the image carrier into a visible toner image with toner that is charged to a polarity opposite to that of the electrostatic latent image. The transfer unit transfers the toner image onto a recording medium or an intermediate transfer member. The cleaning unit removes residual toner remaining on the image carrier so that the now toner-less surface of the image carrier faces the charging unit and can be used repeatedly. Further, residual toner remaining on the surface of the intermediate transfer member is also removed; otherwise, when the toner image is transferred from the intermediate transfer member onto a recording medium in a secondary transfer, residual toner remaining on the surface of the intermediate transfer member can be also transferred onto the surface of the recording medium, resulting in production of background contamination of the final image.
To accomplish the above-described necessary toner removal, blade cleaning as well as conductive or insulating brush roller methods are known. The blade cleaning method for intercepting toner mechanically has been used conventionally since the configuration thereof is simple and inexpensive.
However, in recent years, in accordance with a demand for high resolution and high image quality, toner produced by polymerization has come to be used in place of the conventionally used toner produced by pulverization, since according to polymerization method can uniformly confine toner particle size within a narrow range and moreover high-sphericity particles of can be obtained. In the polymerization method, it is economically advantageous to use toner consisting essentially of spherical particles because their spherical shape requires no deformation processing. Further, toner having a spherical shape enhances transfer efficiency and reduces waste toner amounts, and therefore is more energy-efficient.
The nearly uniform spherical quality of polymerized toner particles has additional advantages. The forces that act on the surface of the image carrier when the toner adheres to the image carrier due to the latent electric potential and a development bias on the image carrier are the mirror force, which depends largely on the amount of electric charge and the distance between charges, and the Van der Waals force. The particles of pulverized toner obtained by conventional pulverization have a concavo-convex surface, whose convex portions are intensively charged by friction. By contrast, since the particles of polymerized toner produced by the polymerization method have a uniformly spherical or near-spherical shape, the surface of each particle is uniformly charged. This is because, in the pulverized toner, toner particles are in close contact with each other at the convex portions, that is, over extended areas, suggesting that much charge is concentrated in a very confined shape so that the mirror force increases. If, however, the toner has a spherical shape as in a polymerized toner, each particle of toner contacts every other particle at a point instead of a plane or other such shape. Therefore the charge amount in such confined regions of close contact is small, and thus the mirror force is weaker than that of the pulverized toner while the Van der Waals force is smaller than that of the pulverized toner.
Thus, in terms of contact force, due to its spherical shape, the particles of polymerized toner (hereinafter also simply “polymerized toner”) has a relatively small attachment force, that is, a small mirror force and a small Van der Waals force, to a photoconductor. As a result, polymerized toner reduces the amount of residual toner remaining after transfer, thus reducing consumption of the toner and therefore provided an economic advantage.
However, when cleaning the residual toner, the polymerized toner, which has a small particle diameter and spherical shape, tends to pass beneath the blade, or put another way, through a gap formed between the edge of the blade and the surface of the image carrier. Therefore, in order to remove the polymerized toner remaining on the image carrier, it is necessary to press the blade against the surface of the image carrier with enough force to intercept the toner. However, strongly pressing the blade against the image carrier accelerates abrasion of both the blade and the image carrier. In particular, to obtain good transferability when an image is transferred onto a recording medium having an uneven surface, an elastic member such as a transfer belt has been gradually come to be used for the transfer member. The ability of the blade to clean such an elastic belt (hereinafter also “cleaning ability”) is unimpaired initially. However, as time elapses, the cleaning ability of the blade is likely to deteriorate.
Moreover, pressing the blade against the image carrier with force necessitates a correspondingly large torque be generated by a motor for driving the image bearing member, necessitating a larger and more expensive motor.
For the reasons discussed above, as an alternative to the blade-based configuration a cleaning method for cleaning a small-sized, peripheral toner without causing damage to the image carrier using an electrostatic brush roller that attracts toner by electrostatic force has been considered.
In related-art cleaning units employing an electrostatic brush make use of the fact that the residual toner after transfer still has an electric charge, and thus the toner is adsorbed and removed by the brush to which a bias voltage of a polarity that is the reverse of the polarity of the residual toner is applied. However, it is to be noted that the electric charge of the residual toner can be either positive or negative by discharging, and is usually both depending on the particle. Since in general the bias voltage applied at transfer step is positive, the negative polarity toner after development is adsorbed and transferred while the positively polarity toner is not transferred and passes through to a post-process.
As described above, discharging occurs between the recording media due to a voltage for transfer, resulting in the co-existence of toner of positive and negative polarities and not simply toner of either one or the other polarity alone. Accordingly, there are related-art devices in which a bias voltage of positive or negative polarity is applied to the toner by use of a bias voltage applying unit such as a brush.
In addition, there is another method in which the polarity is made uniformly positive or negative with the use of a polarity control unit and a voltage reverse to the unified polarity is applied to a cleaning unit to adsorb the toner. The polarity control unit changes the polarity by contacting the toner. It is also to be noted that there is also a non-contact type method in which ions are attached to the outer surface of the toner particles by ionic irradiation such as corona discharge. However, this method has a serious drawback in that the high-voltage discharge generates not only ions but also ozone, which is harmful to humans and to the environment.
In the contact method, the electric charge is controlled by frictional charge, charge injection, or the like, without causing discharge. Specifically, the polarity change is carried out by contacting the toner with a conductive brush, a conductive blade, or the like.
However, in the case of using a brush, the toner tends to adhere to the brush, partially blocking the discharging onto the surface to be biased to unify the polarity and making the polarity uncontrollable over time. In the case of using a blade, the force of contact of the blade against the image carrier causes abrasive degradation in both the image carrier and the blade, which method is therefore problematic in terms of durability.
Consequently, since the blade is originally used to scrape away residual toner, the blade is effectively used to reduce load to the brush when a large amount of residual toner after transfer is conveyed. The toner adhering to the brush is electrostatically conveyed to a toner collection roller that is held in contact with the brush. The toner attached to the toner collection roller is again scraped away by the blade.
Despite the problems associated with polarity control, given its advantages it is not surprising that various types of cleaning units that include a polarity control member have been proposed.
One approach, for example, discloses that a cleaning unit includes a collecting roller to collect toner having a positive polarity and toner having a negative polarity, both of which adhere to a cleaning brush. The collecting roller serves as a high-resistance collecting roller including an electrically conductive member and a surface layer composed of an insulating member formed over the electrically conductive member. The cleaning unit applies a voltage having the same polarity as the voltage applied to the collecting roller to a cleaning member for cleaning the surface of the collecting roller that is held in contact with the surface of the collecting roller. Thus, electrical charge is applied to the surface of the collecting roller. However, the change of polarity of the electric charge applied onto the surface of the collecting roller is to improve the collectivity of toner adhering to the brush roller. Therefore, this technique is not effective for toner the polarity of which cannot be controlled due to deterioration of the polarity control blade.
Another approach discloses that a cleaning unit including a cleaning brush is charged with a voltage opposite to the voltage of the cleaning brush. As a result, both the toner charged to a negative polarity and the toner charged to a positive polarity on the surface of a target body to be cleaned can be attracted to the fibers of the cleaning brush. This approach offers the possibility that toner that passes through the cleaning brush while adhering to the target body but without attaching to the brush fibers can be substantially eliminated, and therefore can prevent poor cleaning. However, this approach does not enhance the cleaning ability and does not improve the durability of the cleaning unit.
Yet another approach discloses an image forming apparatus in which a charge control agent for changing the electrostatically charged polarity of toner, which is reversed due to transfer of a toner image by a transfer roller onto a recording medium, to the same polarity as that of the photoconductor is imparted to an electrostatic charging brush (brush contact) that is held in contact with the surface of the photoconductor. Accordingly, adhesion of residual toner to the photoconductor after transfer is prevented. This can enhance toner collectivity of the residual toner on a developing roller of a developing unit and a constant amount of charge can be maintained for an extended period of time. That is, the cleaning unit collects residual toner to a developing unit by charging to one polarity using a charging member. However, since a brush is used as the charging member, when a large amount of residual toner is conveyed thereto, the cleaning unit cannot collect the residual toner sufficiently, which can result in poor cleaning.
Yet another approach discloses that an image forming apparatus includes a cleaning unit that cleans an electrophotographic photoconductor and that includes at least one brush roller disposed on the surface of the electrophotographic photoconductor and composed of string-shaped brush fibers, each having a surface roughness Ra in a range of 0.15 micrometers [μm]≦Ra≦3.5 micrometers [μm]. Such a cleaning method electrostatically attracts particles of toner having different polarities by using two brushes. However, a gap is formed between the brushes, and therefore a large amount of residual toner remaining after transfer of a solid image cannot be cleaned sufficiently, and even the initial performance may become poor.
The most significant issue of the related-art cleaning unit using the polarity control member described above is poor durability in long-term image forming. The polarity control member is required to scrape toner from the surface of the photoconductor and provide a single polarity, which is a positive polarity or a negative polarity, to toner passing through the cleaning area, and therefore should be robust. However, long-term image forming can cause the edge of the blade to be nicked or gouged, allowing toner to pass under the blade in streaks. The inventors have found that, under these conditions, the toner cannot receive any polarity control, which can result in streaky cleaning defect. Further, another disadvantage is that the toner that has managed to pass under the blade also tends to adhere to the surface of the blade.